]> www.infradead.org Git - users/jedix/linux-maple.git/commit
bpf: Track equal scalars history on per-instruction level
authorEduard Zingerman <eddyz87@gmail.com>
Thu, 18 Jul 2024 20:23:53 +0000 (13:23 -0700)
committerAndrii Nakryiko <andrii@kernel.org>
Mon, 29 Jul 2024 19:53:10 +0000 (12:53 -0700)
commit4bf79f9be434e000c8e12fe83b2f4402480f1460
tree4f8cc132da1332a286cf8f1da75298b1d9c2584f
parent844f7315e77a017fe90652b411e88b119c782a1f
bpf: Track equal scalars history on per-instruction level

Use bpf_verifier_state->jmp_history to track which registers were
updated by find_equal_scalars() (renamed to collect_linked_regs())
when conditional jump was verified. Use recorded information in
backtrack_insn() to propagate precision.

E.g. for the following program:

            while verifying instructions
  1: r1 = r0              |
  2: if r1 < 8  goto ...  | push r0,r1 as linked registers in jmp_history
  3: if r0 > 16 goto ...  | push r0,r1 as linked registers in jmp_history
  4: r2 = r10             |
  5: r2 += r0             v mark_chain_precision(r0)

            while doing mark_chain_precision(r0)
  5: r2 += r0             | mark r0 precise
  4: r2 = r10             |
  3: if r0 > 16 goto ...  | mark r0,r1 as precise
  2: if r1 < 8  goto ...  | mark r0,r1 as precise
  1: r1 = r0              v

Technically, do this as follows:
- Use 10 bits to identify each register that gains range because of
  sync_linked_regs():
  - 3 bits for frame number;
  - 6 bits for register or stack slot number;
  - 1 bit to indicate if register is spilled.
- Use u64 as a vector of 6 such records + 4 bits for vector length.
- Augment struct bpf_jmp_history_entry with a field 'linked_regs'
  representing such vector.
- When doing check_cond_jmp_op() remember up to 6 registers that
  gain range because of sync_linked_regs() in such a vector.
- Don't propagate range information and reset IDs for registers that
  don't fit in 6-value vector.
- Push a pair {instruction index, linked registers vector}
  to bpf_verifier_state->jmp_history.
- When doing backtrack_insn() check if any of recorded linked
  registers is currently marked precise, if so mark all linked
  registers as precise.

This also requires fixes for two test_verifier tests:
- precise: test 1
- precise: test 2

Both tests contain the following instruction sequence:

19: (bf) r2 = r9                      ; R2=scalar(id=3) R9=scalar(id=3)
20: (a5) if r2 < 0x8 goto pc+1        ; R2=scalar(id=3,umin=8)
21: (95) exit
22: (07) r2 += 1                      ; R2_w=scalar(id=3+1,...)
23: (bf) r1 = r10                     ; R1_w=fp0 R10=fp0
24: (07) r1 += -8                     ; R1_w=fp-8
25: (b7) r3 = 0                       ; R3_w=0
26: (85) call bpf_probe_read_kernel#113

The call to bpf_probe_read_kernel() at (26) forces r2 to be precise.
Previously, this forced all registers with same id to become precise
immediately when mark_chain_precision() is called.
After this change, the precision is propagated to registers sharing
same id only when 'if' instruction is backtracked.
Hence verification log for both tests is changed:
regs=r2,r9 -> regs=r2 for instructions 25..20.

Fixes: 904e6ddf4133 ("bpf: Use scalar ids in mark_chain_precision()")
Reported-by: Hao Sun <sunhao.th@gmail.com>
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240718202357.1746514-2-eddyz87@gmail.com
Closes: https://lore.kernel.org/bpf/CAEf4BzZ0xidVCqB47XnkXcNhkPWF6_nTV7yt+_Lf0kcFEut2Mg@mail.gmail.com/
include/linux/bpf_verifier.h
kernel/bpf/verifier.c
tools/testing/selftests/bpf/progs/verifier_subprog_precision.c
tools/testing/selftests/bpf/verifier/precise.c